The present invention relates to an electric mileage totalizer in which a digital means, such as a counter, associated with a sequencer or micro processor, counts the pulses coming from a wheel sensor, translates them and transmits them to a display unit which displays, on the one hand, the total distance travelled since the vehicle was put on the road and, on the other hand, the partial distance travelled since the last reset, this distance information being transferred, in order to safeguard it, into a non volatile memory, each time the supply voltage is switched off.
Non volatile memories offer wide possibilities from the electronic point of view, because of their relatively low cost price and ease of wiring. But they come up against certain problems in use, due more especially to the conditions in which the information is transferred from the counter to the memory and to the limited life span of these memories.
In so far as the transfer of information is concerned, the problem resides in the fact that the counter, which is an integrated logic circuit, for example a micro processor, with a supply voltage threshold, may, whenever it is switched on or switched off, or more precisely at each threshold cross over, generate random signals controlling the memory and possibly causing loss of its contents.
In so far as the life span of the memory is concerned, it depends on the number of possible writing cycles, i.e. on the number of information transfers into the cells of the registers of the memory. At the present time the maximum number is 1000, that is to say that after a 1000 times, writing changes in the cells are no longer guaranteed.
Now, this limitation is very serious for a motor vehicle mileage meter.
In fact, at the rate of ten ignition switch off operations per day for a vehicle used 20 days per month and 11 months in the year, that is to say 2 200 writing cycles per year, a non volatile memory with a maximum number of 1000 cycles would have a life span of six months.
If we assume that the life of a vehicle is 14 years, the writing rate should be reduced by 28 times at least.